Superconducting direct current transmission system

ABSTRACT

A DC superconducting coaxial transmission system provides electrical transmission of 5,000 megawatts of energy while simultaneously delivering liquid hydrogen. The transmission system includes a coaxial transmission segment including an inner superconductor, an outer superconductor disposed in surrounding relation to the inner conductor, and a dielectric insulator disposed between the inner superconductor and the outer superconductor. Liquid hydrogen surrounds the superconductors. Three phase transformers and poly phase rectifier/inverters provide a DC voltage source to the superconductors from the electrical grid. In one embodiment, a switching circuit connected between the voltage source and the superconductor injects a cancellation current component into the direct current flow. The cancellation current component is polarized to flow in a direction opposite to the direct current flow to produce a current zero, thus enabling safe interruption of the high DC current while minimizing the magnitude of transient voltages during switching.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to and this application claims priority from and the benefit of U.S. Provisional Application Ser. No. 61/418,946, filed 2 Dec. 2010, entitled “An Electrical and Hydrogen Transmission System Consisting of Specially Designed Segments”, and U.S. Provisional Application Ser. No. 61/522,456, filed 11 Aug. 2011, entitled “Transmission System for Electricity and Hydrogen Including Specially Designed Segments”, which applications are incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

This invention relates to the field of DC power transmission as a viable means of energy transport over long distances and more specifically, addresses switching and control systems designed to safely deliver energy through the use of superconductor technology.

BACKGROUND OF THE INVENTION

Currently, power transmission in the United States relies on an alternating current (AC) transmission network with three distinctly separate and disparate systems. The failure of any one system cannot be supported by another, and the fact that these networks fail to communicate and interface limits the ability of energy providers to support peak demands and take advantage of regional capabilities and time zones. The existing technology has numerous limitations and drawbacks.

One drawback is that line losses equivalent to I²R heating amounts to 6.2% to 7.0% of the energy carried by the transmission line. Additional losses include dielectric, skin effect, and induction losses. For example, inductive coupling between phases require conductor transposition frequently to compensate for this magnetic interphase coupling. These transpositions result in increased construction costs.

Another drawback is the inability to communicate, support, and coexist in multiple markets. This inability to support distant needs ultimately results in a cascade effect by contributing to uncontrollable pricing fluctuations.

Yet another drawback is that transmission line inductance can and does result in generating a current component, which lags behind the voltage. This component will increase I²R losses and can contribute to system stability problems. This requires expensive and complex solutions to reduce the lagging component magnitude. For example, a long transmission line sometimes requires a large capacitor bank to be installed in series with the line to neutralize the inductance. The longer the line, the greater the inductance, and the greater the size of the capacitor bank required.

SUMMARY OF THE INVENTION

A superconducting transmission system was envisioned which would provide direct current (DC) electrical transmission of 5,000 Megawatts of 50 kA at a DC voltage of 100 kV between conductors, thereby transporting energy while simultaneously moving liquid hydrogen across great distances.

The express purpose in constructing the system utilizing superconducting metal is to extensively reduce the enormous I²R power losses, which would normally occur in a 50 kA conductor. The superconducting material must be chilled at a low temperature to gain superconducting characteristics. The cooling capabilities of various elements at their melting temperatures can provide adequate chilling. Examples of elements, which can be considered, include (but are not limited to) helium (3.2° K), hydrogen (14.2° K), and neon (24.2° K).

The proposed coolant for the present disclosure is liquid hydrogen, chosen because it is a powerful energy source, as well as a coolant. Liquid hydrogen can be pumped through the conductor thereby utilizing the transmission conductor as a means of moving large quantities of hydrogen across great distances.

With the intense interest in future fuel cell powered vehicles, businesses, residences, and other applications, a safe and efficient delivery system for large quantities of hydrogen becomes a need of paramount importance.

One approach to providing an energy transport means is to utilize the disclosed inventive system described herein which will provide for extending the length of a transmission segment up to 1,000 kilometers. This can be done in combination with the utilization of superconductivity to eliminate most of the energy loss existing in AC and DC systems presently in operation. The disclosed coaxial transmission line can be protected by three-phase AC circuit breakers the details of which will be described.

To accomplish such an extraordinary feat, the DC system must also be configured in a very special manner in order to make the interruption of the high DC current possible on the DC side. Furthermore, certain circuit components must be designed to assist in minimizing the magnitude of transient voltages generated in the DC system during switching. Previous DC circuit breakers were of the air magnetic type, which created a current zero by maneuvering the arc produced within the circuit breaker. When the contacts part and the arc moved into a chamber it was elongated and cooled. The cooling and lengthening resulted in a large increase in the resistance of the arc, which introduced a high arc voltage into the circuit. This voltage acted to reduce the DC current to zero, thereby allowing the interruption of the current. This technology is limited to DC circuits with voltage not greater than 2 kV. A new and unique method of producing a current zero is required and will be described.

The inductance of the transmission line must be reduced to the lowest possible value while not sacrificing dielectric integrity. Switching the DC circuit must include multiple circuit breakers, which are capable of interrupting the 50 kA DC current, and isolating the transmission line from the supply or load three-phase AC system at both ends of the transmission line. The system must also include current injection capability—a necessary feature required to produce a current zero in the DC system. Cancellation current is absolutely essential to the interrupting process, which requires that transient over-voltages are minimized.

The invented system to be described will require that the DC current is interrupted regardless of the direction of current flow, or the variation in the magnitude of the DC current. Furthermore, since the conductors must be super conducting, the low temperatures must be maintained by liquid hydrogen, which is, itself, a potential energy source and must be transported over long distances. It is essential to note that the detailed example presented herein describes a 50 kV above and below ground DC system wherein the voltage between the DC conductors is 100 kV. The technology is applicable to higher voltages as dictated by system requirements and development progress on diode and thyristor technology. Higher voltages would likely allow for reduced current values, which would be fully supported by the design technology of the claimed invention.

It is the objective of this invention to assemble the transmission line components into transmission segments, which can be connected together to extend the transmission line length to make possible the delivery of both electrical and hydrogen energy to appropriate distribution locations across great distances.

Before creating the description of the transmission segment it is essential to provide details of the elements which are the basis for each component necessary to support a functioning segment.

The system to be described provides a detailed list and description of interconnected components, which comprise a segment of a hydrogen and electrical DC energy transport line. Segment lengths are selected to match the physical location of the power generation or consumption centers.

In one aspect of the invention, a direct current coaxial superconducting transmission segment is provided. The transmission segment includes a cylindrical inner superconductor defining a core. The core is adapted to flow a first coolant medium. The coaxial transmission segment further includes a dielectric insulator disposed in surrounding relation to the inner superconductor. An outer superconductor is disposed in surrounding relation to the insulator, and a cylindrical housing is disposed in surrounding relation to the outer superconductor. The housing defines a containment for a second coolant medium. The coaxial transmission segment further includes a thermal insulation barrier disposed in surrounding relation to the housing.

In another aspect of the invention, a direct current superconducting transmission system for connection to a three-phase source network is provided. The transmission system includes a coaxial transmission segment including an inner superconductor, an outer superconductor disposed in surrounding relation to the inner conductor, and a dielectric insulator disposed between the inner superconductor and the outer superconductor. A coolant medium surrounds the inner superconductor or the outer superconductor. The direct current superconducting transmission system further includes a first three phase transformer bank connected to the source network. A first poly phase rectifier/inverter is connected to the first three phase transformer bank. The rectifier/inverter is adapted to provide a positive polarity direct current voltage source to either the inner superconductor or the outer superconductor of the coaxial transmission segment. The direct current superconducting transmission system further includes a second three phase source transformer bank connected to the source network and a second poly phase rectifier/inverter connected to the second three phase transformer bank. The second rectifier/inverter is adapted to provide a negative polarity direct current voltage source to the other of the inner superconductor or outer superconductor of the coaxial transmission segment.

BRIEF DESCRIPTION OF THE DRAWINGS

The features described herein can be better understood with reference to the drawings described below. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views.

FIG. 1 depicts a system electrical diagram of DC coaxial superconducting system according to one embodiment of the invention;

FIG. 2 depicts an electrical diagram of a DC coaxial superconducting system including a current injection circuit according to one embodiment of the invention, with power being transmitted from left to right in the diagram;

FIG. 3 depicts an electrical diagram of the DC coaxial superconducting system of FIG. 2, with power being transmitted from right to left in the diagram;

FIG. 4 depicts a detailed view of one of the four injection current circuits shown in FIGS. 2 and 3 and the circuit response;

FIG. 5 depicts another embodiment of a cancellation circuit utilizing a 60 Hz AC current as the cancellation power source;

FIG. 6 depicts a load current interruption utilizing the AC cancellation current source shown in FIG. 5;

FIG. 7 depicts a cross section of the coaxial transmission segment shown in FIG. 1;

FIG. 8 depicts a switching circuit according to another embodiment of the invention;

FIGS. 9 a and 9 b depict an installation plan view and side view, respectively, of the switching circuit shown in FIG. 8;

FIG. 10 depicts an injection current circuit according to another embodiment of the invention;

FIG. 11 depicts an electrical diagram of a superconducting transmission system utilizing a DC capacitor bank as the cancellation power source, with power being transmitted from left to right in the diagram, according to one embodiment of the invention;

FIG. 12 depicts the superconducting transmission system of FIG. 11 with power being transmitted from right to left in the diagram;

FIG. 13 depicts an electrical diagram of a superconducting transmission system utilizing an AC voltage derived from the power transformers as the cancellation power source, with power being transmitted from left to right in the diagram, according to one embodiment of the invention; and

FIG. 14 depicts the superconducting transmission system of FIG. 13 with power being transmitted from right to left in the diagram.

DETAILED DESCRIPTION OF THE INVENTION DC Superconducting Transmission Line Segment Protected by Three Phase AC Circuit Breakers

The proposed objective is to provide a new and unique way of transporting 50 kA DC at a voltage of 100 kV DC while minimizing or eliminating I²R losses through the use of superconductivity. The creation of a transmission segment which combines three-phase AC circuit breakers for protection and isolation with a coaxial superconducting transmission line meets the objective. The disclosed embodiment describes the 50 kA, 100 kV requirements, though choosing higher voltage and reduced current levels are contemplated within the scope of the invention.

Referring to FIG. 1, a direct current superconducting transmission system 10 includes a 5,000 megawatt coaxial transmission segment 12 coupled between terminals of a local electrical grid or network 14 a. The superconducting transmission system 10 includes a 50 kV positive voltage source 16 and a 50 kV negative voltage source 18, which share a common ground 20. The 50 kV positive voltage source 16 is directly connected to an outer positive conductor 22 of the coaxial transmission segment 12, while the 50 kV negative voltage source 18 is connected to an inner negative conductor 24 of the coaxial transmission segment.

Input voltage from the network 14 a is reduced to a line-to-line, non-rectified voltage of 37,065 volts by AC three-phase source transformer banks 26 and 28. The source transformer banks 26, 28 are protected by three-phase circuit breakers 30 and 32 which, when opened together, isolate the coaxial transmission segment 12 from the source network 14 a. Furthermore, the local network 14 a is protected by a three-phase circuit breaker 34 which provides a means of disconnecting the local network 14 a from the transmission segment 12. Each transformer bank 26, 28 must be designed to provide 2500 MW DC, the design MW of the system. The transformer bank MW must be 105% of the DC system MW and provide three-phase RMS line-to-line voltage of 37,065 volts and an AC current of 40,809 amperes. These values, when rectified by a poly phase rectifier/inverter 36, will provide positive polarity DC to the segment connection terminal 16. A second poly phase rectifier/inverter 38 will provide negative polarized DC to the segment connection 18, resulting in a 100 kV difference between the coaxial conductors 22 and 24.

At the load end of the coaxial transmission segment 12, a positive 50 kV DC at a positive load terminal 40 is connected to a rectifier/inverter component 42, which inverts the DC voltage to three-phase AC voltage, which is then transformed to the system voltage by a transformer bank 44 and connected to the load network 14 b by a three-phase circuit breaker 46. The negative 50 kV DC at a negative load terminal 48 is connected to a second rectifier/inverter component 50, which inverts the DC voltage to three-phase AC voltage, which is then transformed to the system voltage by a transformer bank 52 and connected to the next segment by a three phase-circuit breaker 54. The three phase combined output from the transformer banks 44, 52 are connected to the local network 14 b by a second three-phase circuit breaker 56 which provides both protection and isolation from the local network.

The superconducting transmission system 10 includes auxiliary components to facilitate the superconducting properties of the coaxial transmission segment 12. In one embodiment, the transmission system 10 includes vacuum pumping systems 58 a and 58 b, which maintain the dielectric integrity between the coaxial superconductors. Having vacuum sources at each end of the segment 12 provides redundancy. Additional systems and components to the superconducting transmission system 10 may include one or more liquid hydrogen terminals 60, which include refrigeration, storage, and pumping units at each end of the segment 12. This equipment maintains the superconducting properties of the conductors 22, 24 and advantageously transports hydrogen through the transmission segment 12 in a direction dictated by, for example, consumption and hydrogen generation requirements.

The superconducting transmission system 10 may include additional equipment to ensure coordinated functioning of all components within each coaxial transmission segment 12. In one example, a master control complex 62 analyzes data from sensors (not shown) located throughout the entire network. These sensors include, but are not limited to, the measurement of hydrogen temperature and pressure; DC current magnitude and direction of current flow; control status of the solid state rectifier/inverters 36, 38, 42, and 50; and the status of the three-phase protective network 14 a and 14 b and transmission segment circuit breakers 30, 32, 46, 54.

Although not illustrated in the accompanying drawings, the coaxial cable component of the transmission segment 12 must have termination assemblies designed to provide adequate insulation for the high voltage direct current, while allowing for the conductors 22, 24 to be connected to the source and load terminals. Additionally, the vacuum and liquid hydrogen conduits must be connected to external pumping and storage systems by means which are adequately insulated for the high voltage DC.

Circuit for High Voltage DC Switching

Referring now to FIG. 2, wherein like numerals indicate like elements from FIG. 1, in one embodiment of the invention the superconducting transmission system 1010 includes a switching circuit 1064 a to interrupt 50 kA DC at a voltage of 100 kV DC. The switching circuit 1064 a is realized by the injection of a cancellation current component whose polarity and magnitude create a current zero without generating excessive transient over-voltage. In the illustrated example, the switching circuit 1064 a includes two 2-pole circuit breakers and injection current sources which provide a cancellation current component whose amplitude and frequency will produce the desired current zero without generating an unacceptable transient voltage. Although the disclosed embodiment describes a 50 kV above and below ground DC system wherein the voltage between the DC conductors is 100 kV, the technology is applicable to higher voltages as dictated by system requirements and development progress on diode and thyristor technology. Higher voltages for the same transmission capacity as described herein would allow for reduced current values, which are contemplated within the scope of the claimed invention.

The 100 kV-50 kA (5,000 Megawatt) system 1064 a includes a 50 kV positive voltage source 1016 and a 50 kV negative voltage source 1018, which share a common ground 1020. The 50 kV positive source 1016 is connected to the superconductor transmission line positive conductor 1022 by a first circuit breaker 1066 containing dual interrupters 1068 a and 1068 b. In one example, the first circuit breaker 1066 includes a SF6-type hermetically sealed enclosure wherein the interrupter contacts are surrounded by sulfur hexafluoride gas (SF₆) to quench the arc produced during separation of the contacts. The 50 kV negative voltage source 1018 is connected to the superconducting transmission line negative conductor 1024 by a second circuit breaker 1070 containing interrupters 1072 a and 1072 b. The transmission line conductors 1022 and 1024 may be a length in the hundreds of kilometers. The positive conductor 1022 is connected to the load 1040 by a third circuit breaker 1074 with interrupters 1076 a and 1076 b. The negative conductor 1024 is connected to the load 1048 by a fourth circuit breaker 1078 containing interrupters 1080 a and 1080 b. In the switching circuit 1064, I₁ is the current flowing in the positive loop and I₁ is also the current flowing in the negative loop.

Since the two identical components flow through the shared ground conductor 1020 in opposite directions, the net current in the ground conductor is zero and the 50 kA current flows through the positive conductor 1022 and returns to the voltage source through the negative conductor 1024. The transmission line superconductors are coupled utilizing coaxial and magnetic design elements. A bypass switch 1082 may be connected in parallel with each of the circuit breaker poles 1068 a, 1072 a, 1076 a, and 1080 a. The bypass switch 1082 is preferably closed to carry the continuous current and opened prior the interruption by the dual interrupters.

Interruption with DC Cancellation Current Source

As shown in FIG. 2, the cancellation current component I_(C) is initiated by the closure of both poles of the interrupters (e.g., 1068 a and 1068 b) and the subsequent operation of a high current initiating device 1084 when interruption of the load current is required. Because the interruption occurs in two stages, the current I_(C) is always polarized to flow through the circuit breaker contacts (e.g., 1068 a and 1068 b of first circuit breaker 1066) in a direction opposite to the load current I₁ and its frequency and amplitude are selected to produce a current zero, which is necessary to provide a condition wherein current interruption can occur. This current is provided in one example by a first charged capacitor bank 1086 (C) in combination with a first tuning reactor 1088. In stage one of interruption, the magnitude and the rate of rise of the current is determined by the voltage on the first charged capacitor bank 1086, as well as the values of L_(T1), the capacitance of the first charged capacitor bank. The voltage source shown in FIG. 2 is single phase and provides DC voltage to the capacitor bank 1086 from a first full wave bridge rectifier 1090. As an alternative, a three-phase voltage source and a three-phase rectifier can be used to achieve an increased ripple frequency and higher operating efficiency.

Simultaneously, when I_(C) is introduced into the positive circuit (upper portion of FIG. 2), I_(C) is introduced into the negative circuit (lower portion of FIG. 2) when interruption of the current flow is required. The current I_(C) is initiated by the closure of the dual interrupters (e.g., 1072 a and 1072 b) and the subsequent operation of a second high current initiating device 1092. The current I_(C) flows through interrupters 1072 a and 1072 b when the interruption of the load current is required. Because the interruption occurs in two stages, the current I_(C) is polarized to always flow through the contacts 1072 a and 1072 b of the second circuit breaker 1070 in a direction opposite to the load current I₁ and its frequency and amplitude are selected to produce a current zero which is necessary to provide a condition wherein current interruption can occur. This current is provided in one example by a second charged capacitor bank 1094 C in combination with a second tuning reactor 1096. In stage one, the magnitude and the rate rise of the current are determined by the voltage on capacitor bank 1094, as well as the values of L_(T2), the capacitor bank C. The load current positive and negative circuit breakers 1074 and 1078 both open simultaneously with the source breakers 1066, 1070 but no injection of cancellation current is required.

The capacitor banks 1086 and 1094 are charged to the required DC voltage from transformers 1098 and 1100 which are connected to full wave rectifier bridges 1090 and 1102. These transformers also contain an additional secondary winding 1104 to provide voltage for control circuits 1106 and 1108.

It is the objective of the disclosed switching circuit 1064 a to interrupt the load current I₁ by opening circuit breakers 1066, 1070, 1074, and 1078 to completely remove the long distance superconducting conductors 1022 and 1024 from any connection to either the 50 kVDC voltage sources 1016 and 1018 or the loads 1040 and 1048. In the illustrated power system 1010, it is likely that power can flow in either the direction indicated by the I₁ current component or that system requirements demand that current flow in the reverse direction to that shown in FIG. 2. This would mean that the 50 kV voltage sources and the loads exchange location in both the positive and negative portions of the circuit, as shown in FIG. 3. Referring to FIG. 3, to interrupt I₁, the contacts 1076 a and 1076 b of the source positive circuit breaker 1074 open and the operation of the high current capacity initiating device 1110 injects a cancellation component into the contacts 1076 a and 1076 b of the circuit breaker 1074. Simultaneously, in the negative voltage loop, contacts 1080 a and 1080 b of circuit breaker 1078 open and the operation of initiating device 1112 injects a cancellation current into the contacts 1080 a and 1080 b of circuit breaker 1078. The load circuit breakers 1066 and 1070 open simultaneously with the source positive and negative circuit breakers but cancellation current is not required.

The indication of the 50 kVDC voltage sources and loads in FIGS. 2 and 3 are symbolic of electrical interfaces, which connect the three-phase AC systems to the DC transmission system via solid state inversion/rectification circuitry. To determine the values of circuit components, which are essential to the successful completion of the interruption process, the circuits must be analyzed mathematically. Since the positive and negative parts of the circuit are mirror images of each other, the circuit requiring analysis is as shown in FIG. 4. The purpose of the analysis is to provide the designer of the components including the circuit breakers and the superconducting transmission conductors a clear understanding of the requirements. These requirements are centered on the control of the voltage developed by the dI₁/dt within the inductance of the transmission line.

The circuit response of FIG. 4, which correlates to the circuit illustrated in FIG. 2 and utilizes a capacitor bank as cancellation current source, is as follows: After contacts 1068 a and 1068 b have been closed and the bypass switch 1082 has been opened, the DC load current flows unhindered through 1068 a during the period T₀-T₁. Initially, the capacitor bank 1086 is charged to full system positive voltage V_(c). Interrupters 1068 a and 1068 b are both closed prior to time T₁. The graphical representation of the system performance (on the left side of FIG. 4) begins with the cancellation current 1114 (top trace), 1068 a current 1116 (second trace), 1068 b current 1118 (third trace), load current 1120 (fourth trace), and capacitor voltage V_(C) 1122 (fifth trace). Prior to T₁, 1068 a is tripped such that its contacts reach their maximum gap at time T₂. The cancellation current is applied at time T₁ by firing initiating device 1084. The cancellation current forces the load current in 1068 a through zero allowing 1068 a to interrupt the circuit at time T₂. During the period T₁-T₂, the voltage V_(C) is reduced from 100% to zero at time T₂. 1068 a and 1068 b contacts part at different times with the 1068 b contact opening lagging behind that of 1068 a. At time T₂, the voltage on capacitor 1086 is approximately zero, allowing the charging current to flow through 1068 b from the power source 1124 (E) through the capacitor 1086. Capacitor 1086 is now charged to a negative polarity based on circuit capacitance, inductance, and resistance, reaching a partial charge at time T₃ at which time the current through 1068 b reaches zero. 1068 b will now interrupt, thereby isolating the load completely from the power source E power source 1124. After interruption of the load current by 1068 b, the capacitor 1086 can be discharged and charged back to positive polarity, i.e., ready for the next opening operation. The cancellation current magnitude is not adjusted corresponding to the load current flowing in the circuit, but is always at the level which is required to interrupt the maximum rated current of the system (such as 50 kADC).

The mathematical analysis of FIG. 4 is as follows for the period T₁-T₂:

$\begin{matrix} {E = {{L\frac{\mathbb{d}i_{1}}{\mathbb{d}t}} + {RI}_{1}}} & (1) \\ {0 = {{L_{T}\frac{\mathbb{d}I_{C}}{\mathbb{d}t}} + \frac{\int{I_{C}{\mathbb{d}t}}}{C}}} & (2) \\ {{I_{1}(t)} - {{I_{c}(t)}\mspace{14mu}{for}\mspace{14mu}{the}\mspace{14mu}{time}\mspace{14mu}{interval}\mspace{14mu} T_{0}} - T_{2}} & (3) \end{matrix}$

Employing operational calculus, the solutions to equations (1)-(3) are as follows:

$\begin{matrix} {{{I_{1}(t)} = {{\frac{E}{R}\left( {1 - ɛ^{\frac{Rt}{L}}} \right)} + {{i\left( {0 +} \right)}ɛ^{- \frac{Rt}{L}}}}}{{{where}\mspace{14mu}{i\left( {0 +} \right)}} = {{initial}\mspace{14mu}{condition}}}} & (4) \\ {{I_{c}(t)} = {\frac{- E}{\sqrt{\frac{L_{T}}{C}}}\sin\frac{t}{\sqrt{L_{T}C}}}} & (5) \\ {{I_{1}(t)} = {{I_{c}(t)} = {\frac{E}{R} + {\left\lbrack {{i\left( {0 +} \right)} - \frac{E}{R}} \right\rbrack ɛ^{- \frac{Rt}{L}}} - {\frac{E}{\sqrt{\frac{L_{T}}{C}}}\sin\frac{t}{\sqrt{LC}}}}}} & (6) \end{matrix}$

The mathematical analysis of FIG. 4 for time T₂-T₃ can be developed using the techniques described above and including the initial conditions where I₁=50 kA, V_(C)=zero at time T₂ and the total inductance includes the inductance of the transmission line and the tuning reactor. The analysis will yield similar results showing that the current which flows to recharge 1086 will be oscillatory and at a lower frequency.

The determination of the L_(T) and C values is based in part on the velocity characteristics of the circuit breaker contact structure, the interrupter capabilities, and the timing of the cancellation current injection by the controlled operation of the pulse current initiating device 1084. The primary consideration rests with the impact of the cancellation current I_(C), the frequency and the resulting voltage transient generated in the inductance of the transmission line will have on the insulation capabilities of the system. This voltage transient is a direct result of the L dI/dt of the transmission line current and forces the designers of the system to minimize the line inductance and the frequency of the injected current. The invented circuit and the preceding mathematical analysis provide the means of creating an electrical system which will function as a safe and efficient solution to the energy transmission requirements of the United States as well as other countries with similar requirements.

Interruption with AC Cancellation Current Source

The cancellation current source disclosed above utilizes a capacitor bank discharging through a tuning reactor. However, other embodiments of providing a cancellation current source are possible without departing from the scope of the invention. Referring now to FIG. 5, wherein like numerals indicate like elements from FIG. 2, a dual cancellation switching circuit 2064 is provided in which the cancellation current source is AC, such as 60 Hz, for cancellation current derived from the AC side of the described transmission system. This embodiment uses the same SF6 interrupters (e.g., 2068 a, 2068 b, 2072 a and 2072 b) as the cancellation circuit with the DC capacitor bank disclosed with reference to FIGS. 2 and 3, except that the current path through second interrupters 2068 b and 2072 b are provided with a pulse transformer 2126. The purpose of the pulse transformer 2126 is to isolate the cancellation current circuit from the DC load circuit. Secondly, two cancellation current initiating devices 2128 and 2130 and an isolation switch 2132 are required for a successful operation.

The circuit response of FIG. 5, which correlates to the circuit illustrated in FIG. 5 and utilizes a 60 Hz source as cancellation current source, is shown in FIG. 6 and is described as follows: After contacts 2068 a and 2068 b have been closed and the bypass switch 2082 has been opened (the bypass contact structure is synchronized with the opening of the interrupters), the DC load current flows unhindered through 2068 a during the period T₀-T₁. The graphical representation of the system performance for FIG. 6 begins with the cancellation current 2114 (top trace), 2068 a current 2116 (second trace), 2068 b current 2118 (third trace), and load current 2120 (fourth trace). Prior to T₁, 2068 a is tripped such that its contacts reach their maximum gap at time T₂. The cancellation current is applied at time T₁ by firing the first current initiating device 2128. The cancellation current forces the load current in 2068 a through zero allowing 2068 a to interrupt the circuit at time T₂. During the period T₁-T₂, the current through 2068 b increases from zero to 100% at T₂. The 2068 a and 2068 b contacts part at different times with the 2068 b contact opening lagging behind that of 2068 a. During time period T₁ to T₂ the load current transfers to 2068 b and remains at 100% until time T₃. The second current initiating device 2130 is fired at time T₃, shunting the current limiting reactor 2134 and increasing the cancellation current to force the DC load current through zero at time T₄. The contact 2068 b will now interrupt, thereby isolating the load completely from the power source 2124 (E). The cancellation current, which is at a peak at time T₄, will be interrupted by isolation switch 2132 at a subsequent current zero.

High Voltage DC Coaxial Superconducting Power & Hydrogen Transport System

A prior art DC system which was disclosed in a 2006 Scientific American article included two super-conducting cables, each at a potential of 50 kV (one cable positive, and the other negative) relative to ground, and each carrying a current of 50 kA. The superconductors of the prior art cables were described as having an outside diameter of 47.6 cm separated by a center line distance of 78 cm. The inventors of the present invention have analyzed the arrangement to determine its properties, which include the inductance L per kilometer, the magnetic force between the conductors, the stored energy of the pair per kilometer, and the generated transient voltage created during switching.

The inductance L per kilometer of the prior art cable was calculated as follows:

$\begin{matrix} {{L = {{\frac{\mu_{0}l}{\pi}{\ln\left\lbrack {\frac{d}{R} + \frac{1}{4} - \frac{d}{l}} \right\rbrack}} = {504.12\mspace{14mu} µ\; H\mspace{14mu}{per}\mspace{14mu}{kilometer}}}};} & (7) \end{matrix}$

where L=inductance=μ_(Henries/meter);

l=length (meters);

d=separation (cm)=78 cm;

R=conductor radius (cm)=23.8 cm; and

μ₀=4π×10⁻⁷

The magnetic force between the conductors was calculated as follows: F=(B)(I)(Z)=65.38 kiloponds/meter;  (8)

where

${B = \frac{\mu_{o}2\; I}{4\;\pi\; d}};$ I=50 kA; Z=1 meter; and d=78 cm

The stored energy of the pair in 1 kilometer of cable was calculated as follows: E=½(L)(I)²=630,150 Ws;  (9)

where L=504.12 μH per km and I=50 kA

The peak transient voltage generated in 1 kilometer of cable was calculated as follows:

$\begin{matrix} {V_{PK} = {{L\frac{\mathbb{d}I}{\mathbb{d}t}} = {{504.12 \times 10^{- 6} \times 78.5 \times 10^{3}} = {39,573\mspace{14mu}{volts}}}}} & (10) \end{matrix}$

where the current is a sinusoid 318 Hz

The prior art system has several disadvantages, the first being that the calculated inductance L is 504.12 μH per kilometer. This high value results in a stored energy level of 630,150 Ws in only 1 km of cable, and a circuit breaker attempting to interrupt the 50 kA DC current would have to absorb this energy and sustain 39,573 Volts. Another disadvantage of the prior art system is that the magnetic force between the conductors would be 65.38 kiloponds per meter, and the very intense magnetic field surrounding the cables could impose severe unintended consequences on electronic devices in close proximity

The calculations show the extreme difficulty in attempting to switch a cable of only one kilometer in length. Building current interrupters in a system using a 50 kA load level and parallel DC conductors whose length must exceed thousands of kilometers becomes impossible.

The inventors have surmised that part of the solution to this problem is to build a transmission line with a small inductance value and no external magnetic field. The inventors have determined that a coaxial cable design meets this objective. Referring now to FIG. 7, a cross section of the coaxial transmission line superconducting cable 12 of FIG. 1 is illustrated. The coaxial transmission segment 12 includes a liquid hydrogen core 136 a at a temperature of 32 degrees Kelvin. The liquid hydrogen core 136 a is in physical contact with the inner (negative) superconductor 24. The inner superconductor 24 is chilled by the liquid hydrogen to insure that the superconducting properties are maintained. The superconductor cylinder 24 is encased within a non-magnetic stainless steel inner cylinder 138, which acts as a protective coating and as the electrostatic surface of the conductor. An insulator 140 is provided by a vacuum between the inner cylinder 138 and a similarly constructed outer cylinder 142, which is in contact with the outer (positive) superconductor 22. The outer superconductor 22 is also in direct contact with liquid hydrogen 136 b which functions as a coolant medium for superconductor 22. The two conductors 22, 24 are the current carrying members of the transmission line superconducting transmission system 10 illustrated in FIG. 1, the negative conductor 24 being at 50 kV DC below ground potential and the positive conductor 22 being at 50 kV DC above ground potential. The vacuum space 140 will experience an electric stress of 100 kV DC and this space is sized to sustain this voltage as well as any transient over-voltages generated during switching.

The detailed example presented herein describes a 50 kV above and below ground DC system wherein the voltage between the DC conductors is 100 kV. The disclosed technology is applicable to higher voltages as dictated by system requirements and development progress on diode and thyristor technology. Higher voltages would likely allow for reduced current values which would be fully supported by the design technology of the claimed invention.

The liquid hydrogen coolant 136 b which is in contact with the outer superconductor 22 is contained within a non-magnetic cylindrical housing 144. A thermal insulation barrier 146 is provided to reduce heat penetration to an acceptable level. High voltage insulation 148 encloses the entire multi-layer core to eliminate any possible dielectric failure between the positive conductor 22, which is at 50 kV above ground potential, and which is the voltage level of the external stainless housing 150.

TABLE 1 Area ID OD Element Function Material (cm²) (cm) (cm) Thickness  136a Coolant Liquid Hydrogen Coolant 1256.64 — 40.00  24 Superconductor Copper Oxide, Niorium Tin, etc. 522.89 40.00 47.60 3.8 cm 138 Vacuum Stainless steel, non-magnetic — 47.60 48.60 0.5 cm Containment 140 Vacuum space Vacuum — 48.60 53.20 2.3 cm 142 Vacuum Stainless steel, non-magnetic — 53.20 54.20 0.5 cm Containment  22 Superconductor Copper Oxide, Niorium Tin, etc. 522.89 54.20 58.83 2.313 cm   136b Coolant Liquid Hydrogen Coolant 1256.64 58.83 71.14 6.156 cm  144 Hydrogen Stainless steel, non-magnetic — 71.14 72.14 0.5 cm Containment 146 Thermal — — 72.14 99.54 13.7 cm  Insulation 148 High Voltage — — 99.54 105.54 3.0 cm Insulation 150 Protective Outer Stainless steel — 105.54 106.54 0.5 cm Shell

Table 2 provides exemplary properties of the super-conducting coaxial cable 12. Values were derived from the following equations:

Beginning with the inductance per kilometer L for a tube having an outer radius b and an inner radius a,

$\begin{matrix} {{L = {{0.2\; Z\;\ln\frac{b}{a}} = {{0.2\;\ln\frac{27.1}{23.8}} = {{0.2\;\ln\; 1.13866} = {25.97\mspace{14mu} µ\; H\text{/}{km}}}}}},} & (11) \end{matrix}$

where Z=1 meter

Additional defining properties can be calculated once a set of dimensions has been selected. These properties are as follows:

The magnetic force between the conductors was calculated as follows:

$\begin{matrix} {{B = \frac{\lambda}{A}},} & (12) \end{matrix}$

-   -   where B is the magnetic flux density weber/meter², λ is the flux         density=IL, where I is the current in the conductor in amperes         and L is the inductance in henries, and A is the area containing         the magnetic flux in square meters;

$\begin{matrix} {{B = \frac{IL}{A}},} & (13) \\ {{F = {{(B)(I)(Z)} = {{{BI}\mspace{14mu}{where}\mspace{14mu} Z} = {{1\mspace{14mu}{meter}} = {{Newtons}\text{/}{meter}}}}}};} & (14) \\ {F = {1.02 \times 10^{- 1} \times (B)(I)\mspace{14mu}{kiloponds}\text{/}{meter}}} & (15) \end{matrix}$

The stored energy in Watt second/kilometer of cable was calculated as follows: E=½LI ²,  (16)

where L=inductance/kilometer and I=line current in Amperes

A comparison can be made of various coaxial cable designs, as shown in Table 2. When the properties of the inventive coaxial cable are compared with the two parallel DC conductor design of the prior art, it is clear that the coaxial design is a superior choice. Table 2 shows that the stored energy of the No. 4 coaxial design—the details of which are provided in Table 1—is only 5.15% of the parallel conductor stored energy.

TABLE 2 Induct- Induct- Magnetic Stored ance ance Force Energy Design (μH/ (μH/ (kponds/ (watt Energy No. Type meter) km) m) sec/km) % Prior Parallel 0.50412 504.120 65,385.0 630,150.0 N/A Art Cable 1 Coaxial 0.23178 231.080 143.330 288,910.0 45.85 2 Coaxial 0.01613 16.130 131.982 20,162.5 3.20 3 Coaxial 0.01906 19.062 128.42 23,827.5 3.78 4 Coaxial 0.02597 25.970 180.0 32,462.5 5.15

The superconducting coaxial cable 12 shown and described in FIG. 5 and Tables 1 and 2 meet all of the following design requirements:

-   -   a. The cable carries 50 kA to and from the load with minimal I²R         losses;     -   b. The cable is free of an external magnetic field which could         impose severe unintended consequences on the electronic devices         in the vicinity of the cable;     -   c. The conductors must be insulated from each other in order to         sustain the operating voltage, as well as voltage transients,         generated during switching. The evacuated space 140 provides         this insulation;     -   d. The cable is designed with a very low inductance, which is of         primary importance in switching the current off without         generating unmanageable high voltage transients;     -   e. The cable provides a means of transporting large quantities         of liquid hydrogen over long distances; and     -   f. The cable transmission segment presents an aesthetically         pleasing appearance and does not occupy a large area of real         estate, such as is the case of the 765 kV transmission towers         stretching across the landscape mile after mile

The exemplary cable is a 107 cm (42.1 inch) diameter pipeline, and may be painted to blend into the surrounding environment or buried underground.

Circuit Breaker/Pulse Current Initiating Device Description

Referring now to FIG. 8, wherein like numerals indicate like elements from FIG. 2, one embodiment of a switching circuit 3064 is illustrated wherein the interrupters are operated by a common mechanism. The interrupting contact structures 3068 a (+) and 3072 a (−) are enclosed in individual SF6 filled tanks 3066 and 3070, respectively. The interrupters in each SF6 enclosure are controlled by a common operator 3152. In the illustrated example, the source of the cancellation current consists of capacitor banks 3086 and 3094 and high capacity cancellation current initiating devices 3084 and 3092, which in one embodiment are realized as high capacity thyristors.

The capacitor banks 3086 and 3094, though not enclosed, may be mounted on grounded frames and interconnected externally to complete the circuit. The weatherproof compartments enclose the high capacity silicon controlled rectifiers (SCR) 3084 and 3092.

The corresponding dual cancellation switching circuit 2064 adapted for AC cancellation current source was depicted in FIG. 5. This alternative requires a pulse transformer 2126, which is mounted in the SF6 interrupter 2068 enclosure, to isolate the cancellation current circuit from the DC load circuit. As described above, the current limiting reactor 2134, the 2-stage cancellation current initiating device and the isolation switch 2132 are required for the correct operation of the circuit. The interrupter requirements (e.g., 2068 a, 2068 b, 2072 a, and 2072 b) are the same as in the circuit utilizing a DC capacitor bank as the cancellation current source.

FIGS. 9 a and 9 b respectively depict a plan view and a side view of an exemplary installation of the switching circuit described in FIGS. 5 and 8. The installation includes a frame structure 3154 upon which components of the switching circuit are mounted. In one example, the interrupters are hermetically sealed in a SF6-type enclosure 3156 wherein the interrupters are surrounded by SF₆ gas to quench the arc produced during separation of the contacts. The interrupters are coupled by the operator 3152. The installation may include SF₆-to-air bushings 3158 to allow the enclosed switching circuit to be connected to the bare terminals of the source terminal and line terminal. The installation may further include an enclosure or cabinet 3160 for the controls hardware and the initiating devices (e.g., thyristor 3084). Note the pulse transformers 2126 for AC cancellation current source are enclosed in the 3068 b and 3072 b enclosures 3156.

FIG. 10 depicts another embodiment of a switching circuit 4064 and associated response. The circuit 4064 may be utilized in place of the circuit 1064 disclosed in FIG. 2, and the current cancellation source may include the same charged capacitor bank 1086, tuning reactor 1088, and full wave bridge rectifier 1090 disclosed in reference to FIG. 2. The described design utilizes a standard 145 kV−4 kA-63 kA SF6 transmission circuit breaker and a commercially available but modified high speed making switch. One of the three poles of the circuit breaker 4064 is removed and replaced with a high speed making switch 4084 (MS) rated with a minimum of 50 kA DC closing capability at 50 kV dc. One terminal of each of the circuit breaker poles and one terminal of the high speed making switch are connected together and housed in an SF6 enclosure. Depending on the interconnections to the associated equipment (source, line, rectifier) bushings on the making switch and the other end of the circuit breaker may be SF₆-to-air bushings, or SF₆-to-SF₆ bushings.

In this embodiment, the interrupting contact structures 4068 a and 4068 b of circuit 4064 are connected in series and the mid connection between 4068 a and 4068 b is connected to the making switch 4084 by a common SF₆ insulated conductor housed in a tank which is further connected to the two tanks which house 4068 a and 4068 b, respectively. Not shown in FIG. 10 is a bypass switch in a parallel contact structure designed to carry 50 kA continuously. The contacts of the bypass switch open prior to the parting of the interrupting contacts, thus commutating the current into the interrupting structure 4068 a and 4068 b. In all other respects, the operation of the circuit 4064 within the system is the same as embodiments previously described.

The response of the circuit 4064 is shown on the left side of FIG. 10, and is described as follows: Capacitor bank 4086 is charged to a voltage E_(C) sufficient to produce a cancellation current I_(C) 4114 whose magnitude matches the I₁ loop current magnitude. Contacts of circuit breaker 4064 part at time T₀ beginning the interruption process of the load current I₁. The parting of the contacts creates an arc which must be extinguished at a current zero, but time is required both to obtain contact separation and to move the arc into a contact space which can sustain the generated transient recovery voltage at time T₁. Making switch 4084 closes its contacts at T₀ initiating I_(C) current flow through the contact 4068 b in the opposite direction of the I₁ loop current. The magnitude and frequency of the cancellation current I_(C) is dictated by the values of capacitor bank 4086, the tuning reactor 4088, and the source voltage 4124. The I_(C) current 4114 will reach its maximum value at time T₁ and will produce a current zero in the circuit breaker contact 4068 a at which time (T₁) interruption of the I₁ current will occur. The current 4120 flowing through the transmission line conductor 4022 will be I₁-I_(C) during the time interval from T₀ to T₁ and the maximum dI₁/dt will occur at T₀, and will be the slope of the I_(C) sine wave generated by the capacitance 4086 and the inductance 4088. The I_(C) current 4114 will experience a natural current zero at time T₂ and will be interrupted by the circuit breaker contact 4068 b.

The mathematical analysis of FIG. 10 is the same as Equations (1) through (6) presented above.

Description of a Transmission Line Segment

The detailed components of the exemplary superconducting transmission system disclosed herein may now be described in detail as an interconnected system comprising a segment of a hydrogen and electrical energy transport line. The objective is to provide coaxial transmission segments whose lengths can be selected to match the physical location of the power generation or consumption centers across the country. FIG. 11, wherein like numerals indicate like elements from FIGS. 1 and 2, depicts an exemplary superconducting transmission system 5010 and illustrates all segment components as well as the location of each component within the electrical circuit. A complete description is provided for each component and will include the required explanation for the selection of materials and/or dimensions.

In the descriptions to follow, the objective size of a transmission segment shall be 1,000 kilometers (620 miles) in length, but segments of lesser length can be designed as requirements demand Furthermore, the initial description of the operation of the system is based on power being generated on the left side of the circuit shown in FIG. 11 and delivered to the load side (right) of the circuit. Provisions have been included in the invented circuit for power to flow from right to left and this mode of operation is shown and described in FIG. 12.

The description of the primary elements which comprise the building blocks for the creation of a superconducting transmission system clearly defines the capabilities of each component, and includes but is not limited to the aforementioned descriptions in the sections “CIRCUIT FOR HIGH VOLTAGE DC SWITCHING”, “TRANSMISSION LINE SEGMENT”, and “CIRCUIT BREAKER/PULSE CURRENT INITIATING DEVICE”. The operation of the system will now be examined.

Referring to FIG. 11, the input voltage from the AC three phase source transformer banks 5026 and 5028 is reduced to the line to line non-rectified voltage of 37,065 Volts. The source transformer banks are protected by three phase circuit breakers 5030 and 5032 which, when opened together, isolate the segment from the source network 5014 a. Furthermore, the local network is protected by the three phase circuit breaker 5034, which provides a means of disconnecting the local network 5014 a from the transmission line segment coaxial transmission segment 5012. Each transformer bank must be designed to provide 2500 MW DC, the design MW of the system. The bank MW must be 105% of the DC system MW and provide three phase RMS line-to-line voltage of 37,065 V RMS and a current of 40,809A AC. These values when rectified by the poly phase rectifier/inverter 5036 will provide positive polarity DC to the segment connection terminal 5016. The poly phase rectifier/inverter 5038 will provide negative polarized DC to the segment connection negative voltage source 5018. This will result in a 100 kV difference between the coaxial conductors 5022 and 5024.

The transmission line segment 5012 is energized by simultaneous operation of two, 2-pole circuit breakers 5066 and 5070 at the energy source end of the segment and by the simultaneous operation of single interrupter circuit breakers 5074 and 5078 at the load end of the segment. Note that all four circuit breakers must be interlocked to operate simultaneously whether opening or closing.

At the load end of the segment, the positive 50 kV DC at terminal 5040 is connected to the rectifier/inverter component positive rectifier/inverter component 5042 which will invert the DC voltage to three phase AC voltage which is then transformed to the system voltage by transformer bank 5044 and connected to the load network 5014 b by three phase circuit breaker 5046. The negative 50 kV DC at terminal 5048 is connected to the rectifier/inverter component 5050 which will invert the DC voltage to three phase AC voltage which is then transformed to the system voltage by transformer bank 5052 and connected to the next segment by three phase circuit breaker 5054. The three phase combined output from the transformer banks 5044 and 5052 are connected to the local network 5014 b by a three phase circuit breaker 5056 which provides both protection and isolation from the local network.

When circuit interruption is required, the cancellation current must be created and injected into both the positive and negative circuits at the source end of the segment. This process is fully described and selection of the individual circuit components is analyzed since such choices have a direct impact on the ultimate length of the segment.

To determine the values of circuit components which are important to the successful completion of the interruption process, the circuits must be analyzed mathematically. Since the positive and negative parts of the circuit are minor images of each other, the circuit requiring analysis is as shown in FIG. 4, described in detail previously. The purpose of the analysis is to provide the designer of the components including the circuit breakers and the superconducting transmission conductors a clear understanding of the requirements. These requirements are centered on the control of the voltage developed by the dI₁/dt within the inductance of the transmission line.

Examination of the components of FIGS. 4 and 11 are described as follows:

TRANSFORMER—The devices 5098 and 5100 shown in FIG. 11 are depicted as single phase transformers, but would in practice be three-phase units, thereby making polyphase rectifiers available to supply DC to charge the capacitor banks. Furthermore, the size (i.e. kVA) of the transformer is small since it is only required to supply charging current to the capacitor bank. The impedance is not a factor, so the designer can easily provide the necessary insulation to sustain the 50 kVDC plus generated transient voltages previously described. Also, an additional winding can be provided in transformers 5098 and 5100 for control power for the control systems 5106 and 5108 as well as auxiliary devices including the high current making switch (e.g., switch 5084 and 5092).

TABLE 3 Calc. No. E_(C) Freq. (Hz) {square root over (LC)} $\sqrt{\frac{L}{C}}$ LT₂μH C (μF) T₁ (sec) Cap. Vol. (m³) 1 52500 60 2652 × 10⁻⁶ 1.05 2785 2526  4.17 × 10⁻³ 26.174 2 52500 30 5305 × 10⁻⁶ 1.05 5570 5052  8.33 × 10⁻³ 52.348 3 52500 25 6366 × 10⁻⁶ 1.05 6684 6063 10.00 × 10⁻³ 62.824 4 52500 20 7955 × 10⁻⁶ 1.05 8355 7570  12.5 × 10⁻³ 78.532 5 52500 17.5 9095 × 10⁻⁶ 1.05 9549 8662  14.9 × 10⁻³ 89.754 Notes: (1) For calculation No. 3, R = 1155 ohms-21 seconds to charge (2) L = 0.1671/Hz = μH (3) C= 0.90703 ×m L = μF; L is in μH Where E_(C) is voltage on capacitor bank 5086 in FIG. 11 and T₁ is time to I_(C) peak

CAPACITOR BANK—The bank must be constructed using energy storage capacitors because of high current output and the reversal of polarity during discharge. Table 3 presents capacitor bank estimated volume in cubic meters based on manufacturer's data indicating 0.133 Joules per cm³.

HIGH CAPACITY CURRENT INITIATING DEVICE—Several switching devices are selected for initiating current flow from the capacitor bank. A high current making switch which exhibits rapid and consistent closing time and the ability to handle high current magnitudes associated with this application is commercially available. One exemplary make switch available from ETNA Industrie is rated 36 kV rms and 80 kA rms (200 kA Peak) with a closing time of less than 10 milliseconds. An additional solid state device consisting of two banks of back-to-back high capacity commercially available thyristors capable of sustaining current magnitudes involved and can be triggered, and thus provide very precise timing essential to the initiation of the cancellation current pulse.

REACTOR—The inductance of the tuning reactors 5088 and 5096 may be realized by an iron core reactor designed to discharge a capacitor bank whose capacitance value, when matched with the inductance, will produce a frequency and sustain a current magnitude necessary to produce a current zero in the contact structure of circuit breakers first circuit breaker 5066 and 5070. The inductance will also provide a blocking element to voltage transients generated by the dI/dt of the load current in the transmission line during current interruption.

FREQUENCY—Utilization of a low voltage capacitor bank provides a realistic opportunity to examine the advantages of using injection current pulses at frequencies below 60 Hz. The importance of this was recognized in calculations of generated peak transient voltage during interruption of transmission line current.

To complete the design of the coaxial transmission segment 5012, the relationship between the injection current and the segment length must be determined. Table 4 presents segment lengths for several injection frequencies, calculated using the foregoing formulas. In this table, the peak transient voltage (150 kV) generated by the injection current's impact on the load current I₁ which is inducing voltage within the inductance of the coaxial line conductors, a segment length in excess of 1,000 kilometers can be achieved with an injected current frequency of 17.5 Hz. Note that at 17.5 Hz the segment length is increased to 1052.21 kilometers, thereby reducing the number of interconnected segments to five, with six terminal points. This is an enormous improvement and opens many opportunities to the system designers. One can envision a primary transportation system linking San Francisco, Calif., with Boston, Mass. with tap connections extending to metropolitan areas, such as Dallas, Houston, Chicago, and Atlanta, etc.

TABLE 4 LINE SEGMENT LENGTH VS FREQUENCY Transient Voltage Limited to 150 kV Peak Calc Freq. T = 1/f L T/4 di/dt L di/dt = No. (Hz) (ms) (μH/km) (ms) Note 1 (Volts) Segment Length (km) 1 318 3.14 16.13 0.7862  10.0 × 10⁶ 150 × 10³ 93 2 60 16.667 25.97 4.1667 18.84 × 10⁶ 150 × 10³ 306.58 3 30 33.33 25.97 8.333  9.42 × 10⁶ 150 × 10³ 613.15 4 25 40.00 25.97 10.000  7.85 × 10⁶ 150 × 10³ 735.78 5 20 50.00 25.97 12.50  6.28 × 10⁶ 150 × 10³ 919.73 6 17.5 57.14 25.97 14.29  5.49 × 10⁶ 150 × 10³ 1052.21 Notes: ${{(1)\mspace{14mu}\frac{di}{dt}} = \frac{50\mspace{14mu}{{kA}(1.57)}}{T/4}};$ (2) Length = 150,000 / L(di/dt)

The preceding description of the interrupting process was for the positive portion of the source circuit breaker 5064, but the negative circuit for circuit breaker 5070 operates simultaneously and identically with circuit breaker 5064. The critical components, such as the cancellation current initiating devices 5084 and second high current initiating device 5092 and the circuit breakers second interrupter 5072 a and 5072 b are inter-locked to ensure that they will operate together. This is accomplished by the coordinated control components first control circuit 5106 and second control circuit 5108.

Auxiliary components for the coaxial transmission segment 5012 include vacuum pumping systems 5058 a and 5058 b, which are important in maintaining the dielectric integrity between the coaxial superconductors. Having vacuum sources at each end of the segment provides redundancy. Additional components include liquid hydrogen terminals 5060, which include refrigeration, storage, and pumping units and at each end of the segment 5012. This equipment is important in maintaining the super conducting properties of the conductors and in moving hydrogen through the segment in a direction based on consumption and hydrogen generation requirements.

Additional equipment required to ensure coordinated functioning of all components within the transmission system 5010 may include local control units 5106, 5108, 5162, 5164 and a master control complex 5062 which analyzes data from sensors located throughout the entire network. These sensors include, but are not limited to, the measurement of hydrogen temperature and pressure, DC current magnitude and direction of current flow, charge voltage on the cancellation current capacitor banks, the current initiating devices firing circuit status, control status of the solid state rectifier/inverters, and the status of the three phase protective network and segment circuit breakers.

Also, the transmission system 5010 must have termination assemblies (not shown) designed to provide adequate insulation for the high voltage DC, while allowing for the conductors to be connected to the four circuit breaker terminals. Additionally, the vacuum and liquid hydrogen conduits must be connected to external pumping and storage systems by means which are adequately insulated for the high voltage DC.

In the event that the direction of current flow is reversed from the previous description, FIG. 12 depicts the DC current flowing from the source (right side of FIG. 12) to the load side of FIG. 12 (left side). The interrupting process will be precisely as described previously, except for the following changes: the current initiating devices 5110 and 5112 will simultaneously fire to produce a cancellation current component I_(C) in both the positive and the negative portions of the circuit, thereby producing a current zero in the circuit breakers 5074 and 5078. The current initiating devices 5084 and 5092 and will be inter-locked to prohibit their closing since the circuit interruption will be managed at the source end of the segment.

Cancellation Current Pulse Obtained from the 50/60 Hertz Network

As previously stated, the proposed objective of the disclosed invention is to design a circuit to interrupt 50 kA DC at a voltage of 100 kV between the positive and negative conductors. The method described provides a cancellation current pulse whose frequency, magnitude, and polarity will produce a current zero in the load current, thereby creating the environment necessary to allow an SF6 circuit breaker to interrupt the current. The source of the cancellation current pulse has been described in detail as being created by discharging a capacitor bank through a tuning reactor. This approach is desirable because the frequency can be selected to enhance the interruption of the DC current carried by the transmission line whose length, and, therefore inductance, may influence the transient voltage generated during current interruption.

It may be desirable to obtain an alternative cancellation current directly from the 50 or 60 Hz or other frequency network in circumstances where the frequency is not a vital factor in determining the conditions which are essential to insuring a successful interruption of the DC current. Turning now to FIG. 13, wherein like numerals indicate like elements from FIGS. 1, 2, and 5, when power is transmitted from the left side to the right side of a coaxial transmission segment 6012, the superconducting transmission system 6010 can include cancellation current injection components in the positive and negative portion of the circuit consisting of specially designed transformers 6026 and 6028 whose primary windings are fed from the network and whose primary and secondary windings enjoy additional physical bracing and are protected by three phase circuit breakers 6030 and 6032 which, when opened together, isolate the segment 6012 from the source network 6014. Furthermore, the local network is protected by the three phase circuit breaker 6034, which provides a means of disconnecting the local network 6014 from the transmission line segment 6012. Each transformer bank must be designed to provide 2500 MW DC, the design MW of the system. The transformer bank MW must be 105% of the DC system MW and provide three phase RMS line-to-line voltage of 37,065 Volts and an AC current of 40,809 Amperes. These values when rectified by the poly phase rectifier/inverter 6036 will provide positive polarity DC to the segment connection terminal 6016. The poly phase rectifier/inverter 6038 will provide negative polarized DC to the segment connection 6018. This will result in a 100 kV difference between the coaxial conductors 6022 and 6024.

An important part of the circuit is the assembly of the cancellation current pulse circuit components which, in the positive portion of the transmission system 6010, includes current initiating device 6128, isolating vacuum switch 6132, current limiting reactor 6134, reactor shorting switch 6130, and a specially designed isolation pulse transformer 6126, whose windings are arranged coaxially to minimize leakage reactance and whose core is constructed of high permeability magnetic material to insure that saturation does not occur.

At the load end of the coaxial transmission segment 6012, the positive 50 kV DC at terminal 6040 is connected to the rectifier/inverter component 6042 which will invert the DC voltage to three phase AC voltage which is transformed to the system voltage by transformer bank 6044 and connected to the load network 6014 b by three phase circuit breaker 6046. The negative 50 kV DC at terminal 6048 is connected to the rectifier/inverter component 6050 which will invert the DC voltage to three phase AC voltage which is transformed to the system voltage by transformer bank 6052 and connected to the next segment by three phase circuit breaker 6054. The three phase combined output from the transformer banks 6044 and 6052 are connected to the local network by the three phase circuit breaker 6056 which provides both protection and isolation from the local network.

Auxiliary components for the segment 6012 include vacuum pumping systems 6058 a and 6058 b, which are important in maintaining the dielectric integrity between the coaxial superconductors. Having vacuum sources at each end of the segment provides redundancy. Additional components include liquid hydrogen terminals 6060, which include refrigeration, storage, and pumping units at each end of the segment 6012. This equipment is important in maintaining the super conducting properties of the conductors and in moving hydrogen through the segment in a direction based on consumption and hydrogen generation requirements.

Additional equipment required to ensure coordinated functioning of all components within the transmission system 6010, such as a master control complex 6062 which analyzes data from sensors located throughout the entire network. These sensors include, but are not limited to, the measurement of hydrogen temperature and pressure, DC current magnitude and direction of current flow, control status of the solid state rectifier/inverters and the status of the three phase protective network and segment circuit breakers.

The initiation of the positive cancellation current begins in a carefully controlled sequence of events managed by a supervisory controlled system. The load current I₁, which is moving from the left side to the right side of FIG. 13, is interrupted with the opening of circuit breaker 6068 a. When its contacts approach the contact maximum gap at T₂, the current initiating device 6128, which in the illustrated embodiment is a high capacity thyristor bank, will be fired at the preselected crest of the network voltage wave to obtain a symmetrical cancellation current pulse of the proper polarity through the 6068 a contact to obtain a current zero and, consequently, the interruption of the 6068 a current. Initially, the contacts of reactor shorting switch 6130 are open, placing the current limiting reactor 6134 in series with the primary winding of the isolation pulse transformer 6126, but with no voltage applied. The interrupter switch 6132 is initially closed allowing 6130 to apply voltage, reduced by the drop across the reactor 6134, to the primary winding of the pulse transformer 6126. After 6068 a interrupts the current, the load current I₁ is diverted into 6068 b and the secondary winding of 6126 which provides a parallel path to 6068 a. The pulse transformer 6126 is no longer short circuited, therefore, the reduced voltage on the primary winding and the increased impedance, due to the load current inductance on the secondary side of pulse transformer 6126, will produce a reduced component current which is polarized to add to the I₁ current through 6068 b. At the time the AC half cycle comes to zero, the current through 6068 b and the transformer 6126 secondary winding is the DC load current only.

To complete the interruption of the DC load current, 6068 b is given a trip command such that its contacts part at the same time the DC current through 6068 a passes through zero and is interrupted. The switch 6130 is triggered at the first voltage crest, following the interruption of the current through 6068 a producing a large symmetrical cancellation current pulse, polarized to subtract from the load current, thus forcing it to zero and allowing 6068 b to interrupt the circuit. The load current through the transmission line is now completely interrupted.

At the load end of the superconducting transmission system 6010, the network-fed cancellation components in both the positive and negative portions of the circuit include pulse initiating devices 6128, 6130 and reactors 6134 as well as isolating switches 6132 and control systems all of which are not operated because the cancellation pulse is always generated on the source end of a coaxial transmission segment 6012.

When the direction of power reverses to flow from the source, which is located on the right side of the circuit diagram to the left side, FIG. 14 provides a detailed circuit diagram showing all transmission system 6010 components. The interrupting process will be as previously described, except for the following changes: The supervisory control system in the positive portion (e.g., control system 6162) will initiate the operation of current injection components 6128 MS1, 6130 MS2, 6132, and 6134 to produce load current interruption by circuit breaker 6074 which contains SF6 interrupters third interrupter 6076 a and 6076 b. The supervisory control system in the negative portion (e.g., control system 6164) will initiate the operation of current injection components 6128 MS1, 6130 MS2, 6132 and 6134 to produce load current interruption by circuit breaker 6078 which contains SF6 interrupters 6080 a and 6080 b.

While the present invention has been described with reference to a number of specific embodiments, it will be understood that the true spirit and scope of the invention should be determined only with respect to claims that can be supported by the present specification. Further, while in numerous cases herein wherein systems and apparatuses and methods are described as having a certain number of elements it will be understood that such systems, apparatuses and methods can be practiced with fewer than the mentioned certain number of elements. Also, while a number of particular embodiments have been described, it will be understood that features and aspects that have been described with reference to each particular embodiment can be used with each remaining particularly described embodiment. 

What is claimed is:
 1. A direct current superconducting transmission system for connection to a three-phase source network, the transmission system comprising: a coaxial transmission segment comprising an inner superconductor, an outer superconductor disposed in surrounding relation to the inner conductor, a dielectric insulator disposed between the inner superconductor and the outer superconductor, and a coolant medium in surrounding relation to one of the inner superconductor and the outer superconductor; a first three phase transformer bank connected to the source network; a first poly phase rectifier/inverter connected to the first three phase transformer bank and adapted to provide a positive polarity direct current voltage source to one of the inner superconductor and the outer superconductor of the coaxial transmission segment; a second three phase source transformer bank connected to the source network; a second poly phase rectifier/inverter connected to the second three phase transformer bank and adapted to provide a negative polarity direct current voltage source to the other of the inner superconductor and the outer superconductor of the coaxial transmission segment; and a switching circuit connected between the positive voltage source and one of the inner superconductor and the outer superconductor, the switching circuit comprising first and second interrupters and a current cancellation circuit, the current cancellation circuit in parallel with one of the first or second interrupters.
 2. The superconducting transmission system of claim 1, wherein the coolant medium is liquid hydrogen.
 3. The superconducting transmission system of claim 1, wherein the coaxial transmission segment further comprises a second coolant medium in surrounding relation to the other of the inner superconductor and the outer superconductor.
 4. The superconducting transmission system of claim 1, further comprising a first three phase circuit breaker connected between the source network and the first three phase transformer bank, and a second three phase circuit breaker connected between the source network and the second three phase transformer bank.
 5. The superconducting transmission system of claim 1, wherein the switching circuit is adapted for injection of a cancellation current component into the positive polarity direct current, the cancellation current component polarized to flow in a direction opposite to the positive polarity direct current, the frequency and amplitude of the cancellation current component selected to produce a current zero.
 6. The superconducting transmission system of claim 1, further comprising a bypass switch in parallel electrical arrangement with the switching circuit, the bypass switch adapted to continuously sustain a load current through the coaxial transmission segment, and further adapted to commutate the load current to the switching circuit to initiate interruption of the load current.
 7. The superconducting transmission system of claim 1, wherein the current cancellation circuit comprises a making switch and a charged capacitor bank in series with a tuning reactor.
 8. The superconducting transmission system of claim 7, wherein the capacitor bank is charged by a rectifier bridge coupled to the first three phase transformer bank.
 9. The superconducting transmission system of claim 1, wherein the first interrupter and the second interrupter are connected in electrical series, and the current cancellation circuit is connected at a mid connection between the first interrupter and the second interrupter.
 10. The superconducting transmission system of claim 1, wherein the current cancellation circuit comprises a transformer coupled to an alternating current source.
 11. The superconducting transmission system of claim 10, wherein the alternating current source is the source network.
 12. The superconducting transmission system of claim 11, wherein the current cancellation circuit comprises a pulse transformer having a core and primary and secondary windings arranged thereon, the primary winding connected to the first three phase transformer bank and the secondary winding connected in series with the second interrupter contact.
 13. The superconducting transmission system of claim 12, wherein the current cancellation circuit further comprises an isolating vacuum switch, a current initiating device, and a current limiting reactor in series with the primary winding of the pulse transformer, and a reactor shorting switch in parallel with the current limiting reactor.
 14. The superconducting transmission system of claim 13, wherein the primary and secondary windings of the pulse transformer are arranged coaxially to minimize leakage reactance, and the core is constructed of high permeability magnetic material to reduce saturation.
 15. The transmission segment of claim 1, wherein the positive polarity direct current voltage source is 50 kV and the coaxial transmission segment is configured to carry, and the switching circuit is configured to interrupt, a 50 kA DC current.
 16. The transmission segment of claim 1, wherein the first and second interrupters are configured to operate at different times, with one interrupter lagging behind the other. 